1. Field of the Invention
The invention relates to a Darlington transistor arrangement comprising an input transistor having a collector, a base and an emitter and an output transistor having a collector, a base and an emitter, the emitter of the input transistor driving the base of the output transistor.
2. L Description of the Prior Art
Such Darlington arrangements are known for example from Tietze, Schenk, Halbleiterschaltungstechnik, 1978, pages 56-58 and may be employed for example as power transistors in audio amplifiers.
Since the emitter current of the input transistor is equal to the base current of the output transistor when signal current is considered, the current gain factor of such a Darlington arrangement is equal to the product of the current gain factors of the input transistor and the output transistor. When the arrangement is used as a power transistor, in which case the output transistor must be capable of delivering large currents, the base current of the input transistor should not become too large in view of the load presented to the driver stage. Therefore, in order to preclude overloading of the driver stage, the arrangement is generally preceded by an additional transistor whose emitter current is equal to the base current of the input transistor. However, this additional transistor has the disadvantage that a supply voltage which is equal to at least three base-emitter voltages is required for a satisfactory operation of the arrangement. Therefore, the arrangement is not suitable for use with very low supply voltages.
Further, a Darlington arrangement has the problem that the current-gain factor of the output transistor decreases in the case of large currents. This gives rise to a highly non-linear relationship between the output current of the output transistor and the base current of the input transistor, causing distortion of the output signal.